This invention relates to a method of performing an operation in which the effectiveness of the operation is quantified. The invention is particularly, although not exclusively, concerned with the performance of an operation in the form of an inspection procedure such as an inspection procedure on a component of a gas turbine engine in an aircraft.
When a failure occurs in an aircraft in service, steps are taken to understand the underlying cause of the failure so that a plan can be formulated to manage the safety of the fleet (i.e. all other aircraft susceptible to the same failure) against the possibility of reoccurrence. One or more compensating actions will be introduced to keep the fleet flying safely. The entire fleet may be grounded, but the economic consequences of this action are severe, and so it is rarely feasible to ground a fleet until the failed component has been replaced by one which has been redesigned to eliminate the possibility of future failure. Consequently, a common approach is to introduce an inspection task which is intended to have a sufficiently high probability of finding a potential weakness or incipient failure before it becomes a functional failure. This action is often used as an interim measure until a permanent solution such as a component modification is developed. When an inspection task is used as a compensating action, the safety of the fleet (during continued operation) depends significantly on the effectiveness of the inspection as well as on a correct understanding of the failure mechanics and the characteristic life of the component.
The failure probability of a component which is subject to periodic inspection is dependent on two sets of variables: an understanding and numerical quantification of 1) the component characteristic life and 2) the probability of the inspection effectiveness. While formal scientific methods have been developed and are commonly used to derive numerical quantification of component life, inspection effectiveness is generally determined by expert judgement applied in varying degrees and on some occasion by experiment. As a result, errors may arise in the assessment of inspection effectiveness, which can have serious consequences. Also, it is frequently difficult to justify objectively the effectiveness of an inspection.
There are many variables which contribute to inspection effectiveness. The inspection personnel of each fleet operator (or others such as professional non-destructive testing (NDT) companies) who carry out inspection have differing capabilities due to training, experience etc. Effectiveness will also be influenced, for example, by the quality of the inspection equipment available and the environment in which inspection takes place.
An object of the invention is to provide a method of quantifying the effectiveness of an inspection process. Another object of the invention is to provide a method of inspection which includes an evaluation of the effectiveness of the inspection process.
A further object of the invention is to provide a method of assessing the safety of equipment such as an aircraft or an aircraft fleet, in which evaluation is made of the effectiveness of inspection processes carried out on the equipment.
According to one aspect of the present invention there is provided a method of performing an operation comprising operation steps and evaluation steps, the evaluation steps comprising:
identifying a set of performance shaping factors which influence the outcome of the operation;
allocating a probability value to each performance shaping factor, the probability value representing the influence of the respective performance shaping factor on the performance of the operation;
allocating a weighting value to each performance shaping factor, the weighting value representing the significance of the respective performance shaping factor relative to at least one other of the performance shaping factors;
calculating, from the probability values and the weighting values, an effectiveness value representing the effectiveness of the operation.
While a method in accordance with the present invention can be used for a wide variety of operations, it is particularly useful where the operation is an inspection process. In a preferred application of the present invention, the operation is an inspection process carried out on a component of a gas turbine engine.
The operation steps may be carried out before or after the evaluation steps.
If the operation steps are carried out before the evaluation steps, the effectiveness value may be used to vary one or more of the operation steps, or the conditions under which they are carried out, of a future similar operation. If the operation steps are carried out after the evaluation steps, the effectiveness value may be used to determine one or more of the operation steps, or the conditions under which they are carried out. The conditions under which operation steps are carried out may include, by way of example, the nature, qualifications or experience of personnel conducting the respective operation steps, the ambient conditions or the equipment used.
Alternatively, or in addition, the effectiveness value may be employed in conjunction with other factors to assess the safety of continued operation of an aircraft, vessel or other means of transport, or of fixed or moveable equipment such as a power plant, for example a nuclear power plant.
According to a second aspect of the present invention, there is provided a method of assessing the safety of equipment including a component subject to periodic inspection, the inspection comprising inspection steps and evaluation steps, the evaluation steps comprising:
identifying a set of performance shaping factors which influence the outcome of the inspection;
allocating a probability value to each performance shaping factor, the probability value representing the influence of the respective performance shaping factor on the performance of the inspection;
allocating a weighting value to each performance shaping factor, the weighting value representing the significance of the respective performance shaping factor relative to at least one other of the performance shaping factors;
calculating, from the probability values and the weighting values, an effectiveness value representing the effectiveness of the inspection;
utilising the effectiveness value to determine the probability of undetected failure of the component;
determining the consequence of failure of the component in operation of the equipment; and
comparing the probability of undetected failure and the consequence of failure with predetermined criteria and, on the basis of the comparison, either withdrawing the equipment from operational service or continuing operational service.
Preferably, the methods in accordance with the first and second aspects of the present invention are embodied in computer programs, and consequently, in accordance with a third aspect of the present invention there is provided a computer programmed to perform a method in accordance with the first and second aspects of the present invention.